Celestron 1510 Manuel d'instruction - Page 10

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Figure 5

coordinates even though the coordinates denote locations on the celestial sphere, because this makes it

easier to tell how long it will be before a particular star will cross a particular north-south line in the sky.

So, R.A. coordinates are marked off in units of time eastward from an arbitrary point on the celestial

equator in the constellation Pisces.

designated "O hours O minutes O seconds". We call this reference point the vernal equinox where it

crosses the celestial equator. All other coordinates are names for the number of hours, minutes and seconds

that they lag behind this coordinate after it passes overhead moving westward.

Given the celestial coordinate system, it now becomes possible to find celestial objects by translating their

celestial coordinates using telescope pointing positions. For this you use setting circles for R.A. and Dec. to

find celestial coordinates for stellar objects which are given in star charts and reference books.

Polar Alignment

Polar alignment is the process by which the telescope's axis of rotation is aligned (made parallel)

with the Earth's axis of rotation (see figure 7). Once aligned, a telescope with a motor drive will

track the stars as they move across the sky. The result is that objects being observed through the

telescope appear stationary (i.e., they will not drift out of the field of view). If your telescope does

not use a motor drive, all celestial objects in the sky (day or night) will slowly drift out of the field.

This motion is caused by the Earth's rotation. Even if you are not using a motor drive, polar

alignment is still desirable since it will reduce the number of corrections needed to follow an object

and limit all corrections to one axis (R.A.). There are several methods of polar alignment, all of

which work on a similar principle, but are performed somewhat differently.

For each hemisphere, there is a point in the sky around which all the other stars appear to rotate.

These points are called the celestial poles and are named for the hemisphere in which they

reside. For example, in the northern hemisphere all stars appear to move around the north

celestial pole (see figure 6). When the telescope's polar axis is pointed at the celestial pole, it is

parallel to the Earth's rotational axis.

Many of the methods of polar alignment require that you know how to find the celestial pole by

identifying stars in the area. For those in the northern hemisphere, finding the celestial pole is not

difficult. Fortunately, we have a naked eye star less than a degree away. This star, Polaris, is the

end star in the handle of the Little Dipper. Since the Little Dipper (technically called Ursa Minor) is

not one of the brightest constellations in the sky, it may be difficult to locate, especially from urban

areas. If this is the case, use the two end stars in the bowl of the Big Dipper. Draw an imaginary

line through them toward the Little Dipper. They will point to Polaris. The position of the Big

The celestial meridians of longitude are called

"coordinates of right ascension (R.A.)", and like the

Earth's longitude meridians they extend from pole to

pole. There are 24 major RA. coordinates, evenly

spaced around the 360º equator, one every 15º. Like

the Earth's longitudes, R.A. coordinates are a measure

of time as well as angular distance. We speak of the

Earth's major longitude meridians as being separated

by one hour of time because the Earth rotates once

every 24 hours (one hour = 15°). The same principle

applies to celestial longitudes since the celestial sphere

appears to rotate once every 24 hours.

ascension hours are also divided into minutes of arc

and seconds of arc, with each hour having 60 minutes

of arc and each arc minute being divided into 60 arc

seconds.

Astronomers prefer the time designation for R.A.

The prime R.A. coordinate which passes through this point is

Right